Effect of Pressure on the Electronic Structure of Protoporphyrin IX, Hemiporphyrins, and Related Compounds

Studies have been made on the effect of pressure to 170 kbar on the electronic structure of protoprophyrin IX, several hemiporphyrins, and related compounds using optical absorption and Mössbauer resonance. The optical absorption peaks which correspond to $\mathrm{\pi –\pi *}$ transitions shifted to lower energy and decreased in intensity with increasing pressure. This could be explained in terms of changes in the configuration interaction involving a shift of the electrons to the outer portions of the porphyrin ring. Imidazole protohemichrome is a low spin ferric compound. At room temperature and pressures above about 45–50 kbar it reduced apparently to an intermediate spin (or mixed spin) ferrous state. At 110° it reduced at moderate pressure to a low spin ferrous state, but with increasing pressure the Fe(II) transformed to intermediate spin. Hemin and hematin are high spin ferric compounds. With increasing pressure they reduced apparently to intermediate spin (or mixed spin) ferrous compounds. The amount of reduction with pressure was consistent with the location and shift of the observable metal to ligand‐charge transfer bonds. Two ferric compounds with square pyramidal symmetry like hemin and hematin but with planar neighbors of oxygen or sulfur instead of nitrogen were studied. Both reduced with increasing pressure, in one case with a probable change of spin state. All reductions observed were reversible.